In a pressurized-water reactor, a fuel assembly consists of a top nozzle and a bottom nozzle provided with a plurality of openings for the coolant flow. Between the nozzles a number of elongated guide tubes for control rods are arranged. The guide tubes support a plurality of spacers arranged one after the other. The spacers often consist of a number of plates standing on edge and arranged crosswise so as to form a plurality of cells. Fuel rods are inserted into these cells so that the fuel rods together with the control rod guide tubes form an elongated bundle. Unlike a fuel assembly for a boiling water reactor, a fuel assembly for a pressurized-water reactor is not provided with a surrounding casing, but the cooling water which flows in through the openings in a bottom nozzle may be diverted laterally such that it flows over to an adjacent fuel assembly.
Further, it is obvious that the lower the power output from a fuel assembly, the less cooling it needs. Inversely, a higher power may be taken out from a fuel assembly according as its cooling improves. It is also true that the more fuel that is consumed in a fuel assembly, the lower power may subsequently be taken out from the fuel assembly.
From, for example, Swedish Patent 8801141-6 (U.S. Pat. No. 4,994,234) it is known to achieve a control of the coolant flow through a fuel assembly for adaptation to the degree of burnup of the fuel rods. According to this invention, the ends of the elongated fuel rods are adapted to the openings in the top nozzle or the bottom nozzle in order to gradually seal these openings in case of an irradiation-growing linear expansion, thus reducing the coolant flow through the fuel assembly. However, this device does not function very well in a pressurized-water reactor in which the fuel assembly has no surrounding casing. When the coolant flow through, for example, the bottom nozzle is cut and the coolant flow pressure is reduced in the fuel assembly, it may occur that coolant flow with a higher pressure flows over from adjacent fuel assemblies--which, of course, is not desirable.